project: enable DirectX acceleration on iOS

[osy]

Enables the DXMT DirectX-to-Metal backend on iOS, which was previously macOS
only.

• QEMU: the high PCIe MMIO window was silently disabled whenever it did not
  fit in the guest physical address space, leaving nowhere to place a 64-bit BAR
  too large for the 32-bit window. On a host with a small IPA space (36-bit) that
  made hostmem=8G unusable, which is why iOS was stuck on a 256M fallback. The
  window now shrinks to the largest power of two that fits (down to 1G) and warns
  instead of vanishing, so iOS can use the same hostmem=8G as macOS.
• Build: DXMT is now built for the iOS and visionOS sysroots, and
  dxmt-native.framework is embedded in the iOS and UTM SE targets. The DirectX
  backends are loaded at runtime rather than linked, so they moved to embed-only
  and an older device never touches them. A small LLVM 15 patch is included
  because its CMake gates Mach-O linker behavior on CMAKE_SYSTEM_NAME MATCHES "Darwin", which is false when cross-compiling for iOS.
• Deployment target: reverts the minimum version bump that came with the
  original DXMT work. DXMT needs iOS 16 / macOS 13, but that is now handled with
  @available checks at the few places that load it, so UTM keeps supporting
  iOS 14 and macOS 11.3.

Testing: Tested by a human on iOS 16.3.1 and macOS 15.6. The author
acknowledges that this change has been tested and/or reviewed by a human in
accordance with UTM's AI contribution guidelines.